The isolation of single cell clones from bone marrow stroma has enabled the study of molecules which participate in the formation of an extracellular matrix that can sustain the formation of mineral. Cellular properties which include the synthesis of matrix molecules, the mineralization in cultured cells and the extracellular signals that promote these changes are dependent on intracellular processes that occur in response to the signals the cells receive which ultimately result in the formation of a bone matrix. Polypeptide growth factors that bind to their cell surface receptors elicit changes in intracellular proteins and signal the production of specific proteins. The interaction of these proteins with one another and with mineral then results in the production of bone. The bone and collagen fibril forming potential of cell lines marked by transfection with a traceable gene will be analyzed. The cells cloned from bone marrow have a strikingly high response to fibroblast growth factor. Therefore, the marker gene can prove useful in detecting the response of osteogenic cells in vivo to polypeptide growth factors. Extracellular collagen fibrils produced by type I collagen heterotrimer molecules synthesized by a cell with osteoprogenitor characteristics, appear to be distinct from homotrimer collagen molecules that are synthesized by a cloned cell which does not support the formation of extracellular mineral. Our studies will define the properties of transfected cells cloned from bone marrow stroma which differ in their abilities to produce bone specific proteins, collagen fibrils and a mineralized matrix.